Firearm accessories, such as target illuminators, sights and other optics, are typically attached to the firearm using a standardized mounting platform. One common mounting platform is the Picatinny rail, also known as a MIL-STD-1913 rail, STANAG 2324 rail, or tactical rail, although other mounting platforms have been developed for specific applications. These mounting platforms are often attached to some part of the firearm so as to provide for a relatively fixed alignment of the accessory to the bore of the weapon. A number of devices have been developed for mounting accessories to the slide of a handgun, which moves relative to the barrel when the handgun is fired. Due to limits on the manufacturing and design tolerances in the interface between the barrel and the slide, the relationship between the barrel and any accessory mounted to the slide can change with each shot, introducing inaccuracy. Some aiming device accessories have instead been mounted directly to the barrel of the weapon to minimize any relative movement between the bore and the aiming device. However, in some handguns, including semi-automatic pistols, the barrel is encased within the slide, which makes mounting an accessory directly to the barrel difficult.